The present invention relates to electronic devices, and more particularly to bipolar transistors.
Bipolar transistors are widely used in both digital and analog circuits such as computer processors, memories, power supplies, and others. Bipolar transistors are utilized for signal amplification, switching, bias generation, and for other purposes. Because of their high speed, bipolar transistors are widely used in fast memories, fast logic arrays, and many other super fast products for data- and telecommunications.
Typical goals in a bipolar transistor design include a low transistor-generated noise, a high current and power gain (which allow low-power operation), and a high frequency range (which permits a high speed). Another goal is good matching of the electrical characteristics of similar transistors, and in particular good VBE matching measured as the difference between the base-emitter voltages at equal collector currents. VBE matching is particularly important for monolithic circuit design which relies on similar transistors having similar electrical characteristics.
These goals are advanced by reducing the transistor base, emitter and collector resistances and the base-collector capacitance and by increasing the ratio of the emitter area to the base area. The base resistance, in particular, is a major contributor to transistor-generated noise. Moreover, the base resistance decreases the frequency range of the transistor as the transistor unity-power-gain frequency f.sub.max is inversely proportional to the base resistance. See G. Gonzales, Microwave Transistor Amplifiers (Prentice-Hall, 1984) incorporated herein by reference, page 33. The emitter and collector resistances also contribute to noise and, further, they reduce the current gain and the power gain. In addition, the emitter resistance makes good VBE matching more difficult to obtain, particularly at high transistor currents. Further, the emitter and collector resistances and the base-collector capacitance decrease the transistor frequency range as they reduce both f.sub.max and the unity-current-gain frequency f.sub.t. Reduction of the ratio of the emitter area to the base area also reduces the f.sub.T and f.sub.max parameters. See, for example, W. Burger et al., BCTM Proceedings, BCTM 1990, pages 78-81. Thus, reducing the base, emitter and collector resistances and the base-collector capacitance and increasing the ratio of the emitter area to the base area are important goals in transistor design.
Another goal is a small size which is needed to obtain a high packing density in the integrated circuit. In addition, reducing the base size decreases the base-collector capacitance.
FIG. 1 is a plan view of a prior art transistor 110 having three emitters 120-1, 120-2, 120-3 extending across the underlying rectangular base 130. The collector (not shown) underlies the base and electrically contacts the collector contact regions 140-1, 140-2. Low resistivity emitter contact region 150 interconnects the emitters. Low resistivity base contact region 160 overlying the base surrounds the emitters and extends between the emitters. Contact openings 164-1 through 164-6, 170-1 through 170-8, 180-1 through 180-6 in an insulating layer overlying the transistor allow the emitters, the base and the collector to be contacted by conductive layers overlying the insulating layer.
Emitters 120-i are made narrow, that is, the emitter width WE is made small, in order to reduce the intrinsic base resistance RBI, i.e., the resistance to base current of the base portions underlying the emitters. Of note, the base portions underlying the emitters (the "intrinsic base") are typically thinner in the vertical cross section and they are typically lighter-doped than the remaining ("extrinsic") base portion, and hence the intrinsic base resistance is a significant component of the total base resistance. The number of the emitters--three--is chosen to obtain a desired base-emitter junction area in accordance with the desired emitter current. Emitter contact openings 164-i are positioned away from the emitters not to restrict the emitter width--the contact openings are made wider than the emitters to obtain a low contact resistance in the openings. Base contact region portions 160-1, 160-2 extend between emitters 120-i to reduce the base resistance.
Reducing the base, emitter and collector resistances and providing a small size and a low base-collector capacitance are often conflicting goals requiring careful balancing. For example, making the emitters narrow reduces the base resistance but increases the emitter resistance. The base size becomes also increased as more emitters are needed to achieve the desired base-emitter area. Base contact region portions 160-1, 160-2 which extend between the emitters reduce the base resistance but increase the base size. Increased base size leads to a higher base-collector capacitance. The base and collector resistances can be reduced by increasing the base and collector dimensions in the direction along the emitters, but the base size, the base-collector capacitance and the emitter resistance will suffer. Thus, there is a need for a transistor which simultaneously provides low base, emitter and collector resistances, a small base size, a small overall size, and a low base-collector capacitance.
The ratio of the emitter area to the base area in transistor 110 is equal to the ratio n*WE/WB of the total emitter width to the base width, where n=3 is the number of the emitters. This ratio can be increased by increasing the total emitter width n*WE or by reducing the base width WB. Increasing the total emitter width n*WE, however, is undesirable as this would increase the base resistance. The base width WB, on the other hand, cannot be reduced below the limits set by the design rules which require a minimum spacing between emitters 120-i and base contact region 160. Thus there is a need for a transistor having a larger ratio of the emitter area to the base area for the same emitter width and the same design rules.